I haven't dismissed him yet, and don't forsee doing so, even if he's not romanced. The reason I keep him is because I like him. He's a good fighter and easy to get along with while you explore. Most of the companions betray you one way or another, and everyone, no matter how well-regarded, has something they'd rather no one know. Most real people have something bad for a stupid reason and wished they could pretend they didn't do it, too. It's usually insignificant and is forgotten as soon as the blush fades. Since it's a judgement call as to degree when you're talking about issues like forgiveness, it's a good thing his crime is something easy to quantify. It's just plain bad, with no redeeming features. It makes the decision much easier. Either your inquisitor is the forgiving kind or not. I wonder if there's any correlation between pardoning Blackwall and directing Cole's nature? Both dilemmas are about the inquisitor's attitude toward forgiveness.
You were looking for him because he was a warden, and it turns out he wasn't one. He's just a really good fighter who can help the inquisition the same way he has been all along. That much hasn't changed. He comitted mass murder involving young children, motivated by greed. They do go into the motivation behind his greed a little bit, if you talk to him about it, and like most motivation behind doing something awful, it's not really all that great. Point is, look at the other companions. None of them are really squeaky clean, are they (thank goodness.)
Solas is living under an assumed identity in order to avoid the consequences of his crime, which is, strangely enough mass murder, but it involves all the high-ranking members of a rival religion. He didn't blow up the conclave, he just let Corypheus. That was why he was in the neighborhood trying ineffectively to fix what he let loose. He doesn't have the courage to admit his identity or the gravity of his crime, even after he criticizes Blackwall for it, he just leaves and lets you find out about it later from someone else. But he has the temerity to criticize Blackwall. And the presumption to give advice to Cole. I always send Cole to Varric. Fen'Harel may know more about being a spirit in the real world, but he's a jerk and I get tired of listening to him once I've found enough artifacts to get the bonus rift. By the time we meet Corypheus he's hanging out in his office wearing his smallclothes and waving a broomstick.
Varric is the Thedas equivalent of an old-time mobster, even down to Bianca, which he holds like a tommy gun. He tells stories and is easy to get along with, reminding me of several people, but his black market stuff was what started the whole mess with the red lyrium. He's a smooth talker and you do never know if he's telling the truth or protecting someone, who or may not deserve protection. Point is, as lovable and familiar as he is, he has a very criminal past, and his mess is also biting Inq's butt.
Vivenne is using her position of power within the Inquisition to maneuver into assuming control of the Chantry. She tells you her political connections up front, but not how extensive they are until you've brought her (difficult to obtain) ingredients for an elixir that is probably of morally questionable origin. Her kinda sister-in-law, the grand cleric, says the Circle declared Bastien's illness incurable, so Viv had to find the recipe from a source outside the Circle. So, she condemns the use of magic outside circle sanction, while using it herself, meanwhile placing herself in position to control what is to be sanctioned in the future. Right. If you've got nothing against a scheming dictator being in charge of the largest political body in Thedas, while not being shy about working outside it if the need suits her, she's your gal. She'll even betray her own people to ensure her view of the world is adopted (she's a mage and favors a return to the system that started the mage war.)
Sera's an agitator who wants to upset the world order to make it better for the ones on the bottom. I don't agree with how effective her pranks are, but at least they are (mostly) harmless to the people she's trying to help. The problem with her is that she's really limited in scope and can't see how sometimes her way of doing things makes things worse, not better. Those kitchen elves might not have been getting pushed around by the cook if she hadn't tried to fix it with her pranks, for instance.
If you don't like Blackwall, that's fine. I think the writers did such a great job portraying everyone, you've got plenty of reasons not to get along. He's coarse, not the most cunning man in the world, and has a past he's lied about. The thing is, you get the chance to tell him what you think and send him away if you want, or just leave him to rot and never even come for him in jail, in which case, he'll pay for his crimes. It's not as if he's the only warrior. If you like, you can even just keep him as long as you need him and send him to the wardens when you're done with him, after the game. You get enough information to make that decision, and there are even different consequences for each. I just don't see how there can be any one right decision.